We, and all the other mammals, reptiles amphibians and birds, are
descended from a variety of fish very closely related Coelacanths
- unless you are a "Creationist" or "Intelligent Designer" - in which
case God saved a lot of time designing you by simply kluging his
successful fish design, resulting in some pretty funky engineering. So,
in either case, what's inside a fish is pretty much the same as what's
inside you - just arranged differently. Coelacanths were long
thought extinct, but were rediscovered in 1938. They are characterized
by four sturdy leg-like fins.
Photo by FASTILY, distributed under license Creative Commons
Attribution Share-Alike v3.0 Unported.

This diagram is typical of ray finned bony fish, the vast majority of
fish species, and almost all that appear in our fish markets. Ray finned
fish appear in an incredible array of sizes and shapes, including eels,
but are all made of the same components in the same general arrangement,
just exaggerated in different ways. This diagram does not cover sharks
and rays, which are not closely related. It also doesn't fit the bone
structure of lobe finned fishes (our ancestors) which have arm and leg
bones, but you are unlikely to encounter any of those.
Illustration by Marrabbio2 contributed to the Public Domain (liver
added).

When the modern bony, ray fined fish (Teleostei) appeared, the older
types were under great pressure due to being eaten by the new fish. One
branch of the older fish responded by very rapidly evolving in two
directions - adopting some characteristics of its own predecessors, but
also evolving forward well beyond the Teleostei.

Advanced features include a bigger, more complex brain, allowing a
much more sophisticated social structure, internal fertilization and live
birth, and an array of very sensitive sensory organs for variations in
water pressure, sight, smell and minute electrical charges. Some even
developed a degree of warm bloodedness. So successful were these
adaptations, the following era is called "The Age of Sharks", and it was
the Teleostei who had to worry about getting eaten to extinction.
Illustration by Chris_huh, contributed to the Public Domain
.

The shark Skeleton is not bony, but cartilaginous. This
relatively primitive structure is light weight, allowing sharks to
become quite large. They can also get along without a swim bladder,
enabling more vertical freedom in the water column. Actually, sharks
have negative buoyancy and their swimming is more like flying. If not
swimming or resting on the bottom, they will sink.

Shark Scales are actually teeth, very hard and difficult to
penetrate. Many cultures use shark skin for sandpaper and graters.

Shark Teeth are structured the same as their scales. They
are in several rows, and the front teeth are continuously replaced
over the life of the shark. Their teeth are shaped for ripping, not
chewing.

Shark Blood contains a large amount of urea so as to be in
osmotic balance with seawater, so most sharks can't survive in fresh
water, but some with modified kidneys can. When a shark dies, bacteria
degrade the urea into ammonia, so sharks must be well bled when caught,
and eaten or frozen in a short time. Do not buy shark or skate meat that
smells strongly of ammonia, it is not fresh.

Shark Tails are quite asymmetrical, because the spine curves
up into the top lobe, allowing powerful propulsion and high swimming
speed.

Rays and Skates are sharks who's pectoral fins have
become so large they are like wings. These sharks are mostly, but not
entirely, bottom dwellers. Rays give live birth, while Skates lay
eggs. The giant Manta Rays are deep water filter feeders. Once feared,
scuba divers have shown them to be harmless and even friendly.

Temperature: Most sharks are cold blooded, but some of
the most active predators can keep their body temperature above ambient
for more vigorous activity than cold blooded fish can manage.

Most sharks give Live Birth to fully developed young up to
2 feet long and able to fend for themselves. Some nourish their young
by placental attachment like mammals. Others use a different system
where some nourishment comes from the mother, but the first young to
hatch also eat unhatched eggs, and sometimes each other. This produces
few but relatively large well developed offspring. Rays and some
smaller sharks have returned to producing eggs, generally in leathery
sacks.

Shark Brains are similar in size compared to body mass to
birds and some mammals. Most tend to be quite social, and display
curiosity and play behavior similar to birds and mammals.

The Spiracle is a special gill that feeds oxygen directly
to the eyes and brain. Teleostei fish do not have this feature, but
then they hardly have brains.

In case you are curious as to what your remote ancestors looked like,
here is one of their relatives that is still alive today. Coelacanths
date from about 400 million years ago, and were thought long extinct
until science was shown living examples in 1938. Note that the pectoral
and pelvic fin pairs are on stems, which contain the jointed bones which
eventually developed into the arms and legs we enjoy today. These fin
stems are anchored to well developed internal bone structures. Some
species were quite lizard-like in shape, as is the related (and still
living) Queensland Lungfish. Living lobe finned fish are noted for
possessing either vestigial or functioning lungs. All in all, they are
more closely related to amphibians, reptiles and mammals than they are
to the ray finned bony fish.